Casual viewers dont really care about the character ages. Its simply one thing that modern audiences like - they want to see young and impossibly pretty actors in their movies, although in fact the actors are nowhere near as young as their counterparts - John Cho is roughly 20 years older than Sulu - although Pegg and Urban are about the right age.

My personal preference would have been to leave Chekov out of the first movie and use one of the missing women instead. Janice Rand could have slotted in quite nicely as a security guard or Pike's yeoman (or both). I understand why they wanted to use Chekov straight away but they leave Robin out of all the early Batman movies. Cramming all the big 7 into the first film was not all that necessary. If Chekov was missing, fans would have accepted it in a heartbeat and non-fans would not care either way.